Troops should not be transferred directly from Iraq to Afghanistan after a "significant reduction" is made to the British contingent there next year, the head of the armed forces said today.

Sir Jock Stirrup, the chief of the defence staff, said it was crucial for the armed forces to experience a reduction in "operational tempo" in order to recover from several years of being overstretched.

His comments came as the foreign secretary, David Miliband, indicated that the UK, whose 8,100 troops in Afghanistan make it the second largest contributor to the international force, would expect other Nato countries to take up a bigger share of the burden in any US-led "surge".

The US president-elect, Barack Obama, is preparing to order a significant increase in troop numbers in Afghanistan in the hope of quelling the Taliban insurgency in the country.

Miliband said this would "not necessarily" require an increase in the size of Britain's commitment there.

"As the second-largest contributor of troops in Afghanistan, the first thing we say is that we don't want to bear an unfair share of the burden. The second thing we say is that more foreign troops on their own are not going to provide the answer in Afghanistan," Miliband told BBC1's Andrew Marr Show.

Obama has also pledged to withdraw US troops from Iraq by 2012. The majority of British troops are expected to return home by the middle of next year. No official timetable has been set but an official announcement is expected by Christmas.

Stirrup said he was "optimistic" that 2009 will see a "significant reduction" in the UK's 4,000-strong contingent in Iraq, as the fundamental change of mission promised by Gordon Brown comes into effect.

But he cautioned that this will not mean that thousands more servicemen and women become available for deployment to Afghanistan.

"I have said for a very long time that the British armed forces are stretched. We are doing more than we are structured and resourced to do in the long term. We can do it for a short period, but we can't continue doing it ad infinitum," Stirrup said.

"We have to put ourselves back into balance. It is crucial that we reduce the operational tempo for our armed forces.

"So it can't be - even if the situation demanded it - just a one-for-one transfer from Iraq to Afghanistan. We have to reduce that tempo."

He warned that the struggle in Afghanistan was "a marathon, not a sprint", adding: "We need to be there at the finishing line."

He said he was cautious about the so-called "surge" being seen as a panacea but said it was clear more military force was needed in Afghanistan.

"In the context of what we are doing in Iraq and Afghanistan, we are shouldering a burden which is more than we are able to shoulder in the long term, so we expect the others to take up their share of that burden," said Stirrup.

The shadow foreign secretary, William Hague, said he believed the bulk of British troops would be pulled out of Iraq next year and also warned that Britain made a "disproportionate contribution" to the Nato effort in Afghanistan.

He told Sky News: "I think by this time next year, British troops will have been pulled out of Iraq, other than some who may remain for training and other special operations. But I think the bulk of the troops will have been pulled out and we will be supportive of that in the opposition."

He added: "We do need the rest of Nato to play its part in Afghanistan and undoubtedly it seems that Barack Obama does intend to send larger US forces and that is part of what is necessary in Afghanistan," said Hague.

Miliband and Stirrup both ruled out the commitment of British troops to the peacekeeping force in the Democratic Republic of the Congo. "That is not on the agenda," said Miliband.